Various additives have been proposed for inhibiting the discoloration, degradation, and/or embrittlement which occur in polymers, such as polypropylene, on exposure to irradiation. Suppression of these deleterious changes in the polymers is necessary to render them suitable for use in articles which are to be sterilized by irradiation, a sterilizing technique commonly practiced on medical articles.
U.S. Pat. No. 4,110,185 describes incorporating a low molecular weight, preferably not highly viscous, liquid mobilizer, such as a hydrocarbon oil, into a semi-crystalline polypropylene to increase the free volume of the polymer and allow it to retain its flexibility after irradiation. U.S. Pat. No. 4,274,932 describes incorporating the mobilizer in a semi-crystalline polypropylene which has been visbroken to give it a narrow molecular weight distribution.
According to U.S. Pat. No. 4,749,734, a mobilizing additive, such as a hydrocarbon oil, and a heterocyclic hindered amine are used in combination to stabilize semi-crystalline polypropylene against radiation. A thioester should not be present.
U.S. Pat. No. 4,710,524 notes that the inclusion of a mobilizing additive as described in U.S. Pat. Nos. 4,110,185 and 4,274,932 produces some undesirable side effects including handling and imprinting problems. U.S. Pat. No. 4,710,524 discloses stabilizing polyolefins against the effects of radiation by incorporating therein a hindered piperidine compound and a light-stabilizing additive, such as a benzophenone precursor.
U.S. Pat. No. 4,563,259 describes the addition of from 0.01 to 2.0%, by weight of the polymer, of the polymer of a hindered amine or its salt, N-oxide, N-hydroxide or N-nitroxide to a substantially crystalline polymer of an aliphatic C.sub.2-6 monoolefin.
U.S. Pat. No. 4,797,438 describes using hindered amines containing a polysubstituted piperazin-2-one moiety without an antioxident to achieve improved strength and discoloration resistance in irradiated propylene polymers.
U.S. Pat. No. 4,888,369 discloses inhibiting the radiation-induced degradation of polypropylene by dispersing therein, prior to irradiation, a synergistic mixture of a hindered amine component, a hindered phenolic component, and a phosphorus-containing component. The polypropylene has a narrow molecular weight distribution, achieved by treating ("visbreaking") polypropylene having its as-prepared molecular weight distribution.